An transmitter optical module applied in the optical communication system as an optical signal source provides an semiconductor laser diode (hereafter denoted as LD) and an optical coupling system to couple the light emitted by the LD with the optical fiber through at least one lens. A receiver optical module also applied in the optical communication system provides a semiconductor photodiode (hereafter denoted as PD) and an optical coupling system to couple the light provided from the optical fiber with the PD through at least one lens. Moreover, a bi-directional optical module has been known, which includes both the transmitter optical module and the receiver optical module, and may perform the optical transmission and the optical reception for the single fiber.
Conventional optical modules described above usually provide the optical coupling system between the optical fiber and the semiconductor device to be, what is called, the parallel beam coupling system, that is, the light output from the LD, which is divergent light, is collimated by a first lens, and focused on an end of the fiber by another lens, a condenser lens. This coupling system may secure an enough distance between two lenses; accordingly, other optical components such as an optical isolator and so on may be set between two lenses.
FIG. 13 schematically illustrates a parallel beam coupling system of a conventional optical module 100 that installs two LDs and one PD each processing light with a specific wavelength different from others. These three optical devices, 117A, 117B and 119B, communicate with the signal fiber 113 through individual lenses, 122a, 122b and 122f, and a condenser lens 121. The light emitted from the LD, 114a and 114b, is collimated by an individual lenses, 122a and 122b, multiplexed by a WDM filter 124a and condensed by the condenser lens 121 on the end of the optical fiber 113; while the light provided from the optical fiber 113 is collimated by the condenser lens 121, reflected by the second WDM filter 124d, and focused on the surface of the PD 115b by the individual lens 122f. 
The individual lenses, 122a to 122f, are installed in respective optical devices, 117A to 119B. Because the optical module 100 has the parallel beam coupling system, the optical alignment along the optical axis of respective optical devices, 117A to 119B, may be roughly carried out, and only the alignment in a plane perpendicular to respective optical axes are precisely performed by sliding the optical device, 117A to 119B, on the outer wall of the coupling unit 111.
FIG. 12B estimates the optical coupling loss when the condenser lens is offset from the optical axis in the optical system shown in FIG. 12A, where a distance between two lenses is set to be 5.00 mm, a working distance from the end of the fiber to the lens is set to be 1.762 mm, and another working distance from the LD to the lens is set to be 0.297 mm. Offsetting the condenser lens in the plane perpendicular to the optical axis, the optical coupling loss monitored through the optical fiber is evaluated. FIG. 12B shows that the loss degrades more than 0.5 dB for an offset of only 5 μm; and for the offset of 3 μm, the coupling loss increases to around 0.2 dB.
FIG. 11B shows a result of the same estimation with those shown in FIG. 12B for the focused beam coupling system of FIG. 11A where a distance between two lenses is set to be 5.13 mm, a working distance from the lens to the optical fiber is set to be 2.84 mm, and another working distance from the other lens to the LD is set to be 0.27 mm. In the focused beam coupling system, the coupling loss is less than 0.6 dB even when the offset of the lens increases more than 50 μm, and the coupling loss less than 0.2 dB is allowed for the offset of about 30 μm, which means that the focused beam coupling system makes it possible to facilitate the optical alignment between the optical fiber and the optical devices, even when an optical module is necessary to install a plurality of optical devices for the single fiber.